Benefits
Supports hydration by retaining more fluid than the same drink alone
Glycerol taken with a large drink lowers urine output so more fluid stays in the body. A pooled analysis of 14 comparisons found about 7.7 mL more fluid retained per kg of body weight than with fluid alone, roughly 540 mL at 70 kg. One heat study found plain water raised body water just as much. These data come from plain glycerol.
Endurance exercise performance in the heat
Results are mixed. Small trials found longer cycling to exhaustion and 5% more work in a hot cycling time trial, and a pooled analysis of 4 trials found about 2.6% better endurance performance, which its authors said needs more study. Other trials found no change in 60-minute cycling, a 5-km run in the heat or a half-marathon with glycerol plus salt.
Fluid retention when combined with sodium
In a resting crossover study, glycerol alone retained about 729 mL over 3 hours, salt alone about 1,127 mL and the two together about 1,435 mL. In a half-marathon trial, glycerol plus salt raised fluid retention by about 1,189 mL versus the same fluid volume but did not change race time.
Heart rate and body temperature during exercise
One early trial reported a heart rate about 3 to 4 beats per minute lower after glycerol, but core temperature was not lowered in most trials, including studies at 32 to 35 C, and one trial found a higher heart rate during hard efforts. A pooled analysis of hyperhydration studies found no significant overall effect on heart rate or body temperature.
Mechanism of action
Osmotic fluid retention
Glycerol drinks create an osmotic gradient in the circulation that favours fluid retention, so less of a large drink is lost as urine.
Less urine from the kidneys
In a resting study, glycerol cut urine flow and free water clearance compared with water alone. Changes in antidiuretic hormone (ADH) were small and near the limits of the test, and the authors suggested glycerol may also raise the concentration gradient in the kidney so more water is reabsorbed.
Clinical trials
Meta-analysis of studies comparing glycerol hyperhydration (on average about 1.1 g/kg with 24 mL/kg fluid) with the same fluid without glycerol (Goulet et al. 2007, Int J Sport Nutr Exerc Metab)
Pooled studies: 14 comparisons for fluid retention, 4 for endurance performance.
Glycerol increased fluid retention by about 7.7 mL per kg body weight, roughly 540 mL at 70 kg. Endurance performance improved by about 2.6%, a marginal result (P = 0.047) from only 4 studies, and the authors said more research is needed before drawing firm conclusions about performance.
Randomized, crossover, counterbalanced study at rest comparing salt (7.5 g/L), glycerol (1.4 g/kg fat-free mass) and both together, each in 30 mL/kg fat-free mass of fluid (Goulet et al. 2018, Int J Sport Nutr Exerc Metab)
15 young men.
Fluid retained after 3 hours was about 729 mL with glycerol, 1,127 mL with salt and 1,435 mL with both; urine output was lowest with the combination. Plasma volume changes did not differ significantly. There was no plain-water arm, and abdominal discomfort was low in all three.
Double-blind, repeated-measures crossover trial of glycerol 1 g/kg in a diluted carbohydrate-electrolyte drink versus the same drink alone (22 mL/kg total fluid) at 32 C and 60% humidity (Hitchins et al. 1999, Eur J Appl Physiol Occup Physiol)
8 competitive cyclists.
Glycerol expanded body water by about 600 mL over placebo and increased work done in the variable-workload half of the time trial by 5%. Rectal temperature, sweat rate and heart rate did not differ between trials.
Crossover trial of glycerol 1.2 g/kg with 21 mL/kg flavoured water versus the same volume of water, 2.5 hours before a self-paced 60-minute cycling test in hot, humid conditions (Marino et al. 2003, Pflugers Arch)
7 moderately to well-trained adults.
Null result: distance cycled (28.9 km glycerol vs 29.7 km water), power output and final rectal temperature did not differ. Heart rate was higher with glycerol during the high-intensity efforts.
Randomized, counterbalanced trial of glycerol 1.4 g/kg fat-free mass with 30 mL/kg fat-free mass of cold water versus a smaller water drink, before a 5-km running time trial at 30 C (Jolicoeur Desroches et al. 2023, Nutrients)
10 recreationally active adults.
Glycerol increased fluid retention (846 mL) and plasma volume, but 5-km time (22.95 vs 22.52 minutes), heart rate, gut temperature and perceived exertion did not differ significantly.